1. Field of the Invention
The invention relates to a method of crystallizing amorphous silicon. More particularly, the invention relates to a mask for crystallizing the amorphous silicon, a sequential lateral solidification (SLS) crystallizing method and a manufacturing method of an array substrate using the same.
2. Discussion of the Related Art
Due to rapid development in information technology, display devices have evolved into instruments that can process and display a great deal of information. Flat panel display devices, which have properties of thinness, low weight and low power consumption, such as liquid crystal display (LCD) devices, have been developed. The LCD device is widely used for notebook computers, desktop monitors, etc. because of its superior resolution, color image display and quality of displayed images.
In general, the liquid crystal display (LCD) device includes two substrates, which are spaced apart and facing each other, and a liquid crystal layer interposed between the two substrates. Each of the substrates includes an electrode, and the electrodes of each substrate are also facing each other. Voltage is applied to each electrode and an electric field is induced between the electrodes. The alignment of the liquid crystal molecules is changed by varying the intensity or direction of the electric field. The LCD device displays a picture by varying the transmittance of the light varying according to the arrangement of the liquid crystal molecules.
One substrate of the LCD device includes a thin film transistor that acts as a switching device. Amorphous silicon is widely used as an active layer of the thin film transistor because amorphous silicon can be formed on a large, low cost substrate such as glass.
The LCD device also includes a drive integrated circuit (drive IC) that controls the thin film transistor. Unfortunately, amorphous silicon does not form a suitable active layer for the drive IC, which usually includes CMOS (complementary metal-oxide-semiconductor) devices that require crystalline silicon as active layers. Because of this, the drive IC is usually connected to the array substrate using a TAB (tape automated bonding) system. This adds significant cost to the LCD device.
Because of the limitations of amorphous silicon, an LCD device that incorporates polycrystalline silicon as an active layer is being researched and developed. Polycrystalline silicon is highly beneficial because it is much better suited for use in the drive IC than amorphous silicon. Polycrystalline silicon thus has the advantage that the number of fabrication steps could be reduced because a thin film transistor and a drive IC could be formed on the same substrate, eliminating the need for TAB bonding. Furthermore, the field effect mobility of polycrystalline silicon is 100 to 200 times greater than that of amorphous silicon. Polycrystalline silicon is also optically and thermally stable.
Polycrystalline silicon can be formed by depositing amorphous silicon on a substrate, such as by plasma enhanced chemical vapor deposition (PECVD) or low-pressure chemical vapor deposition (LPCVD), and then crystallizing that amorphous silicon into polycrystalline silicon. There are a number of different methods of crystallizing amorphous silicon into polycrystalline silicon, including solid polycrystalline crystallization (SPC), metal induced crystallization (MIC), and laser annealing.
In SPC, a buffer layer is formed on a quartz substrate. Then, amorphous silicon is deposited on the buffer layer. The amorphous silicon is then heated at a high temperature, over 600 degrees Celsius, for a relatively long time. The buffer layer prevents impurities from diffusing into the amorphous silicon. The high temperature causes the amorphous silicon to crystallize. However, the SPC method results in irregular grain growth and non-uniform grain size. Therefore, gate insulating layers grow irregularly on SPC-formed polycrystalline silicon. This decreases the breakdown voltage of the resulting TFTs. Moreover, the electric properties of the TFTs are reduced because of the irregular grain sizes. Additionally, quartz substrates are expensive.
In MIC, a metal deposited on amorphous silicon induces crystallization at a relatively low temperature. This has the advantage that lower cost glass substrates can be used. However, the deposited metals remaining in the silicon layer act as detrimental impurities.
In laser annealing, an excimer laser irradiates an amorphous silicon layer on a substrate for several tens to several hundreds of nanoseconds. This causes the amorphous silicon layer to melt. The melted silicon subsequently solidifies into polycrystalline silicon. In the laser annealing method, crystallization can be accomplished at less than 400 degrees Celsius. Unfortunately, crystallization is relatively poor, particularly if the silicon layer is crystallized using a single laser shot. In practice, re-crystallization is usually performed by irradiating the laser beam about 10 times or so to increase the grain size. Therefore, laser annealing suffers from low productivity. Furthermore, laser irradiation can heat the silicon layer to about 1400 degrees Celsius. Because such temperatures would readily oxidize the silicon layer to produce silicon dioxide (SiO2), laser annealing is usually performed at room temperatures and in air or atmosphere.
Recently, a new method of crystallization, often referred to as sequential lateral solidification (SLS), has become of interest. The SLS method takes advantage of the fact that silicon grains grow laterally from the boundary between liquid silicon and solid phase silicon. The SLS method can increase the size of the silicon grains that grow by controlling the energy intensity of a laser beam and the irradiation range of the laser beam (see Robert S. Sposilli, M. A. Crowder, and James S. Im, Mat. Res. Soc. Symp. Proc., Vol. 452, 956–957, 1997). This enables TFTs having channel areas of single crystalline silicon.
A related art SLS method will be described in detail with reference to the attached drawings. FIG. 1A shows a mask for a sequential lateral solidification (SLS) method according to the related art and FIG. 1B illustrates a silicon layer crystallized by using the mask of FIG. 1A.
As shown in FIG. 1A, a mask 10 for a SLS method includes a slit 12 of several micrometers, and thus makes a laser beam incident on an amorphous silicon layer have a width of several micrometers. Here, a space between adjacent slits 12 also is several micrometers, and the slits may have a width within a range of about 2 μm to 3 μm.
If a laser beam is irradiated on an amorphous silicon layer 20 of FIG. 1B through the slit 12 of the mask 10, a region 22 exposed to the laser beam is completely melted, and then is solidified to form grains 24a and 24b. At this time, the grains 24a and 24b grow laterally from both end lines of the region 22, and stop growing at the line 26 where the grains 24a and 24b meet each other. The line 26 may be referred to as a grain boundary. Here, the mask 10 includes a plurality of slits 10, and a crystallized region corresponding to a size of the mask 10 may be referred to as a unit region.
Next, a laser beam is irradiated on a next region including the region 22 of FIG. 1B, and then the next region is crystallized. The amorphous silicon layer 20 may be wholly crystallized by repeatedly performing the above-mentioned processes.
FIG. 2 shows a part of a silicon layer crystallized by the above method. In FIG. 2, the crystallized silicon layer includes multiple unit regions 30, and there are first and second overlap regions 40 and 50 between adjacent unit regions 30. The first overlap region 40 is disposed between the adjacent unit regions 30 in a horizontal direction (in the context of FIG. 2) and the second overlap region 50 is formed between the adjacent unit regions 30 in a vertical direction (in the context of FIG. 2). The first and second overlap regions 40 and 50, where the laser beam is irradiated several times, include non-uniform parts. Therefore, in the case where the first and second overlap regions 40 and 50 are disposed in a pixel area of a liquid crystal display device, the quality of displayed images decreases.